It is proving harder for NASA to explore Earth than to send spacecraft millions of kilometers to Jupiter. Launches of two important Earth observation satellites are again on hold due to technical problems, hobbling researchers' efforts to gather data on everything from land-use changes to the carbon cycle and the role of clouds in blocking the sun's radiation.

Landsat 7 and EOS-AM1 were slated for launch earlier this summer, but glitches in both pushed the launch schedule to the end of this year. Now NASA officials say neither will be ready for orbit until next year. During a recent thermal vacuum test, a large batch of key components failed on the $444 million Landsat 7, according to James Irons, the project's deputy scientist.

Meanwhile, problems with the flight operations software for the $1.2 billion EOS-AM1, the first major spacecraft in the Earth Observing System series, likely will set that launch back until next summer, according to agency officials.